It is known from the specification of European patent application No. 0 678 410 to provide an air inlet for a motor vehicle which includes a main valve of the drum type, which selectively obturates a fresh air inlet or a recirculated air inlet, together with a pressure adjusting valve, the purpose of which is to generate dynamic pressure in the fresh air. The pressure adjusting valve is coaxial with the main valve, and is again of the drum type. Pivoting movement of the two valves is controlled by a single drive device which cooperates with control levers so as to provide cam operated control of the displacement of the valves. In another version, the two valves may be controlled separately.
Controlled displacement of the pressure adjusting valve enables the dynamic pressure at the fresh air inlet to be held substantially constant regardless of the flow velocity of the air at the fresh air inlet, since this velocity is a function of the speed of forward travel of the vehicle and of the ambient wind speed.
The coaxial arrangement of the two drum type valves has the result that the multi-valve device occupies a great deal of space, and this is a disadvantage due to the fact that the rules and practices of vehicle design tend always to greater and greater economy of space.
Another disadvantage of the device described in the above mentioned patent is that, when the main valve is operated so as to obscure the recirculated air inlet, the valve which generates dynamic pressure partly covers the fresh air inlet. Similarly, when the main valve is operated so as to cover the fresh air inlet, the dynamic pressure generating valve, i.e. the pressure adjusting valve, will in general momentarily occupy a position in which it partly covers the recirculated air inlet. As a result, it is necessary in each case to withdraw the pressure generating valve entirely into the interior of the main valve in order to open an air inlet fully.
In the case where the two valves are controlled by a single mechanism, the driving device has to be able to provide sufficiently precise positioning of the pressure adjusting valve. Such a device, for example a motorised reduction unit with a stepping motor, does not enable the valves to be operated with a high pivoting speed.
In the other case, in which the two valves are controlled separately, the pressure adjusting valve can only be controlled by a relatively light mechanism, for example a step-by-step motorised reduction unit, which means that the above mentioned withdrawal of the pressure adjusting valve into the interior of the main valve cannot be carried out at high speed.
In either of these two cases, the time for full opening of one or other of the air inlets is unable to be optimised.